The present invention relates to an empirical two-step technique for sensing the oil/water ratio (cut) utilizing a small unit that is in-line in a fluid transport piping system or in a bypass mode outside the main transport system.
Water occurs naturally in hydrocarbon formations and is used in the secondary recovery of oil from wells. Because of the increased value of crude oil extracted with a lower content of water, it is important to maintain the highest possible oil/water ratio. Without timely measurements of the oil/water cut at the wellhead, effective production activities are difficult to ascertain. Therefore, the measurement of the oil/water cut is important to the efficient recovery of oil.
Historically, oil/water cut measurements have relied upon taking samples, separating the oil and water, and measuring the volumes of the separated components. This can be done at individual wells by taking samples at a wellhead or at a block of wells by measuring the water in the separating tank.
It has been proposed to measure the water content in an oil/water mixture by measuring the electrical properties of the mixture. By far, the majority of the techniques measure capacitance rather than resistance. The primary difference with each technique lies within the specific electrical properties which are being measured; the overall methodologies and apparatus are similar (often identical) and similar problems with accuracy arise, as will be discussed below.
One technique is to pass the oil/water mixture between two parallel plates (test cell). Using the test cell, the dielectric constant of the oil/water mixture is determined by measuring the dielectric capacity. Using the same test cell, one could measure the resistance (or conductance) of the mixture. This technique is inaccurate because the electrical measurement requires an unusually accurate knowledge of the physical dimensions of the test cell which must remain fixed when used in the field if recalibration of the device is to be avoided.
For capacitance measurements, the change in capacitance with water content, particularly at low ambient temperatures, is so small that the sensitivity of the measurement is inadequate. This problem has been addressed by placing a water-sorptive, solid dielectric material in contact with the fluid and determining its electrical loss characteristics. Because any practical water-sorptive material displays an hysteresis with changing water content, again, continuous recalibration is necessary, severely limiting the usefulness of this device.
An extension of these techniques is one in which a grid pattern of crossed vertical and horizontal wires is used to determine the location of clusters of conductive water in a nonconductive fluid, thus implying the water content. The spacing of the wires determines the spatial resolution. For a heterogeneous mixture of oil and water, there is a problem in establishing the water content.
These techniques are most accurate in the middle range. When either oil or water dominate the mixture, the sensitivity of these measurements decreases dramatically. However, for end member ratios, capacitance measurements are more accurate than measurements of resistance.
Non-electrical methods depend upon differences of physical properties. Those methods utilizing the differences in physical properties usually require taking a sample and allowing the oil and water to separate. Although accurate, they are essentially an extension of techniques that have been in use for many years and suffer from the limitations mentioned below.
Another method depends upon measuring gamma rays resulting from capture of thermal neutrons. For the gamma/neutron techniques, the fluid is bombarded with fast neutrons which become thermalized and are then captured by materials in the fluid mixture; these materials then emit high energy gamma rays. The intensity and spectra of the gamma rays allow one to estimate the oil/water cut. Differences in neutron absorption are induced by the varying composition of the oil/water mixture. Periodic certification and great care in handling the neutron source are required. This technique is variable and very expensive.
One unusual technique measures sonic velocities of a flowing oil/water mixture from which the oil/water cut may be determined. As with the electrical techniques, this technique requires careful attention to the influence of environmental parameters such as ambient and oil/water temperatures.
In all of these methods, the information is not timely. In those where measurements are made at a separating tank, only a gross indication of where the problem actually exists is given. Thus, a method is needed that can sense the oil/water cut at the wellhead or at another location within the pipeline that is accurate and can be measured within whatever time frame is desired. A method which allows remote readings will enhance the overall economy and usefulness of the technique. The output from such a device should be easily and immediately transmittable to a convenient location. This would allow one to quickly assimilate an accurate picture of the state of a project.